Adding a circuit that is hot during run only

[curtis73]

I want to figure out a way to make a circuit that is only hot when the engine is running. Like, as it is, there is one side of the fuse panel that is hot and one side that is switched. I want one that is only hot after the engine is running. I'm building a car right now with some heavy draw items like a compressor for air suspension, a couple amps for a medium duty stereo, DRLs, and they normally would come on with the switch. How does one go about adding a run-only circuit so those items wait until the start is done and the alternator is making power?

[sierrap615]

i use a "fuse-tapper" or some name like that, it takes the spot of a mini fuse, and has two fuses mounted to it, one for the original circuit, one for a new circuit, and it has a wire coming out of it. it only holds upto ten amps, so i still tap the battery and use a relay. i picked it up at autozone or advance auto or someplace like that.

tell me if you have any trouble finding this thing

[curtis73]

Thanks but I'm talking about a completely different thing. The fuse panel only has two choices; either hot while the switch is on, or hot all the time. I want an entirely different thing. I want an electrical source that is only hot when the engine is actually running. I'm not talking about adding a new circuit to run fog lights, I'm talking about a whole new set of juice.

[benchtest]

Two possibles come to mind. 1) If you never leave your key in the 'run' position with the engine not-running, then you could find a tap that is 'run' only and trigger a relay off of that, or 2) you could build/buy (if available) a circuit that would trigger a relay based on voltage...say over 13.5 volts would turn it on.

[sierrap615]

ah i see, you could tap into the charge indictor light, but be carefull if the light is controlled by the PCM

[curtis73]

Good suggestions. I'm somewhat knowlegable with electronics, but I'm far from an engineer. I know diodes only let juice go one way, but I don't know anything about their operating parameters. What I was thinking was a Diode between the alt and battery, then connect a relay to the alt. That way, when the alt is making power, the relay is active, but when the alt stops, no juice flows back to it. The problem is; its a 140-amp alt. How does one design a diode that is capable of that type of amperage?

[psychorallyfreak]

Here's an idea, run your circuitry off of a key-hot source, then run it through a switch before it hits the rest of the circuit. Or switch to a relay to the circuit...
You'll get the idea.

[sierrap615]

naw curtis, good idea, but a diode on the alternator pos cable isn't practical. i was thinking about it today, i would find the wire to charge/battery light, put a LED inline(minimum increase in impedance). then line it up to a photodiode, seal the two up so no stray light gets in, then wire it to a N.C. relay that is hot with key on. that way the light goes off, with the key on, the relay is closed. light off key off - relay not powered. light on key on - relay open.

i think thats right..... i gotta go to bed...... :zzz:

[curtis73]

Excellent idea! Now the problem is... The car is gutted of the entire interior and wiring. The old system used something in the external voltage regulator, but that's been bypassed since 1991. How does one make a charge light ? Once I get that charge light circuit figured out, I can do it.

[public]

Vacuum controled switch?? Get one from Greybar.

[curtis73]

Oooh. Now we have our thinking caps on! I also thought about a tiny microphone epoxied to the block, but I think the heat and vibration would get to it in short order.

[public]

Old time bicycle generator running like hell off of the drive belt

[sierrap615]

Originally Posted by public

Vacuum controled switch?? Get one from Greybar.

never knew that existed, i like that idea

[RandomTask]

Relay possible?
are you running an aftermarektt ECM? A lot of aftermarket ones have one or two extra 5V in and out for controlling things you set up. These can usually be set up in analogue or digital.
Curtis, I'm gonna head down to the race shop and ask our Electrical Engineer for a sure fire way to do this, just gimme one day =)

[curtis73]

Cool. Tell him that its a serious old school car, so I have no ECM. Its a caddy 500 and the only "electronics" are the electric choke and the HEI ignition. Its old school

Thanks